I'm shooting a film in which I have to get footage of a bird in flight (from underneath and against the sky). I am considering using a captive bird for this but really need a "natural" look so would have to remove (in post) anything the bird might have to wear around its ankles, for example what the bird is wearing in this clip:

Would it be possible to do this seamlessly in post (perhaps using After Effects or possibly an alternative) or would it be obvious that effects were used?

If a seamless method is possible, would it extremely time-consuming (or expensive if someone is hired) to remove the unwanted elements in say 10-30 seconds of footage?

If you're shooting against the sky, it shouldn't be too hard to mask out. It depends on how much the strap thing (I don't know what to call it) is flapping around. At worst, it would take about 30 seconds to a minute per frame of just rotoscoping I think (less or more depending on the skill of the person doing it). Then you may have to spend some more time replacing the keyed element, depending on the complexity of the BG. If youre using a long lens and the bird is pretty high, I think it would not be that difficult.

Someone else would have to comment on cost of hiring some, I don't really have that kind of knowledge.

An alternative would be stock footage. If you can find a good shot, it might save you a lot of time.